The search for new sources of hydrocarbon fuels has led to the development of novel deposits of naturallyoccurring hydrocarbon material. Among these are the bituminous deposits of northern Alberta in Canada. Taken together, the hydrocarbons present in the McMurray Formation, the largest of these deposits, is estimated as equivalent to 800 billion barrels of crude oil. Because of limitations on surface mining as now practiced, only the top 20 feet or so of tar sand can be mined. Even so, it has been estimated that this portion alone contains the equivalent of 200 billion barrels of crude oil. The principal advantage of the mining route is that it allows bitumen to be extracted from mined tar sand by the highly efficient hot water extraction process, wherein mined tar sand is agitated with steam and water, and sometimes such process aids as sodium hydroxide, and the resulting slurry is advanced to a separatory vessel where much of the bitumen floats as a froth and coarse sand sinks to the bottom and is discarded as a valueless tailings stream. Commonly a middlings stream that takes up an intermediate position in the separatory vessel and that contains typically in the region of 2.25 weight percent bitumen, but in such a form as to be unable to float, is withdrawn and a further yield of bituminous froth obtained therefrom, usually by the forced addition of air. The most common means of isolating the bitumen from the froth streams is to mix the combined froth with a naphtha solvent to produce a mixture of bitumen dissolved in naphtha as well as water and mineral solids and then to centrifuge the resulting mixture. Such centrifuging is commonly performed in two stages, first, using a degritting or scroll centrifuge machine to remove the larger-sized mineral particles, and secondly, in a high speed disc machine to take out substantially all the remaining mineral solids and water leaving a relatively pure solution of bitumen in naphtha solvent. The solvent may then be recovered by flash distillation. The process has been well described in the patent and other scientific literature.
The hot water process and other extractive methods applied to mined tar sand typically extract 93% of the bitumen. This compares very favourably with in-situ methods which, as presently practiced, may recover around 40% of the bitumen.
A less commonly recognized advantage arising from the use of the mining route but one that is involved with the present invention is that it allows isolation or concentration of the heavy minerals, present in the sand of the formation, whereas with in-situ techniques, such minerals remain on the ground. Although the composition of tar sand varies throughout the deposit, tar sand from the McMurray Formation may be said to typically analyze at 11.59 weight percent bitumen, 4.41 weight percent water, 84.00 weight percent mineral solids. Again speaking generally, the more interesting of the mineral solids commonly include quartz (silica), clay, corundum, rutile, ilmenite, leucoxine, zircon, kyanite, apatite, aluminosilicates, garnet, amphiboles, feldspar, monazite, and mica. This list is not necessarily complete for all areas of the deposit.
The minerals fall into groups according to density. The light minerals of density up to 3.0 are principally silica sand (SiO.sub.2), ferric oxide (FeO), and ferric carbonate (FeCO.sub.3). Those whose density ranges from 3.0 to 4.0 are mostly iron aluminum silicates. The rest of the minerals (ranging in specific gravity from 4.0 to 4.6) contain the zirconium-based and titanium-based minerals of commercial interest. These are mostly ilmenite (TiO.sub.2.FeO), leucoxine (2TiO.sub.2.FeO), rutile (TiO.sub.2), and zircon (ZrSiO.sub.4). Of these, the titanium and zirconium minerals are of commercial value after suitable concentration by a beneficiation process, and in fact, the hot water extraction may be looked upon as a first step in heavy minerals' beneficiation.
In froth treatment, most of the heavy minerals report to the tailings from the first-stage or scroll centrifugal separator with the result that such tailings typically analyze at:
8 to 12% iron by weight PA1 5 to 9% titanium by weight PA1 2 to 5% zirconium by weight. PA1 Sieving through a 20 mesh screen PA1 Re-slurrying with water PA1 Treating in a hydrocyclone PA1 Adjusting the water content to give a slurry of 25 to 50 weight percent solids PA1 Concentrating heavier material by a gravity separation in water (for instance by the use of Humphreys' spirals) PA1 Drying PA1 Separating into zirconium-rich and titanium-rich fractions under high tension voltage PA1 Cleaning and concentrating the zirconium-based and titanium-based minerals by selective magnetic separation. PA1 stream low in heavy minerals, largely silica PA1 stream relatively rich in titanium minerals PA1 stream relatively rich in titanium and zirconium minerals PA1 stream low in heavy minerals, largely clays.
Unlike the free-flowing beach sand used as feed in the conventional heavy metals beneficiation process, for instance in Australia, centrifuge tailings from tar sand extraction are a sticky mass impregnated with bitumen and water.
We have determined that water and organic material may be removed from the centrifuge rejects by a burnoff process. This process is described in U.S. Pat. No. 4,138,467, issued Feb. 6, 1979 which is incorporated herewith by reference. The mechanism, as it is conjectured to occur, may best be described as a 2-stage process. In practice however, it is not necessarily thus carried out. In the first stage (coking), the scroll tailings are introduced into a fluid bed reactor and under an inert atmosphere of nitrogen are heated to 1025.degree. F. or thereabouts. This treatment removes volatiles, including water, probably by a mechanism that includes (a) driving off light hydrocarbons (b) driving off moisture (c) "cracking" some bitumen to gaseous hydrocarbons that are then driven off under the influence of the nitrogen stream (d) "cracking" some bitumen to liquid hydrocarbons that are not volatile under the reaction conditions (e) converting some bitumen to a carbon coke that adheres strongly to the mineral particles. The hydrocarbons that are evolved from the scroll tailings in the fluid bed reactor may be cooled in a condensor and thus recovered. Secondly (burn-off stage), while the fluid bed reactor is at 1025.degree. F. or thereabouts, external heating is discontinued, the nitrogen is switched off, and air or oxygen is fed to the reactor. This causes oxidation of the carbon which excapes from the reactor as gaseous oxides of carbon. In commercial continuous operation the above treatment would most probably be preferably performed in a single step, for instance in a Herreshof or other open hearth furnace.
Such treatment yields a free-flowing product of mineral solids which is an appropriate feed stock for further concentration steps. Such steps may be:
The present invention notably simplifies beneficiation after the burn-off process thus leading to simplified operation and lower investment without increased losses of the desired minerals.
In the concentrating process for heavy minerals described above, the separatory steps involving water depend upon gravity differences in the various mineral fractions. Taking advantage of this principle the dense particles are successively concentrated and the lighter material rejected. The process carries the disadvantage that it involves expensive drying steps and a relatively large investment in equipment, with attendant complexity in the operation of such equipment.